Phosphazene polymers are valuable structures in several industries including environmental cleanup, mineral processing and potentially in industries such as dentistry, the aerospace industry and the military to name a few. As a specific example of use, phosphazene polymers can be made into semipermeable polymer inorganic membranes which are useful in the separation of desired components from fluid mixtures, e.g., U.S. Pat. No. 4,749,489. In fact, polyphosphazene membranes are more versatile than corresponding organic polymers used for similar purposes. Specifically, polyphosphazene membranes may be used at high temperature as well as in caustic and acidic environments. These characteristics are valuable in many areas including petrochemical separations, mineral processing, catalytic reactions and environmental cleanup. Conversely, organic membranes only remain stable in conditions of up to about 90.degree. C. and in relatively benign environments.
As a second example illustrating the use of polyphosphazenes, there is promise in the area of dental surface protection. Research has been conducted indicating that polyphosphazenes may be effective in the protection of dental surfaces from acidic attacks causing demineralization, and that as denture liners they offer many advantages. These and other examples of the use of polyphosphazenes illustrate that these polymers are a valuable asset to several industries.
Polyphosphazenes are a broad and well known class of macromolecules that come in various forms. The linear polymer is essentially phosphorus and nitrogen joined by alternating single and double covalent bonds with additional groups also covalently bonded to the phosphorus. A representation of this polymer is as follows: ##STR1##
In the above structure, n may be a positive integer from 3 to 10.sup.8 and X and Y may or may not be the same substituent. For example, if the polymer were poly(dichlorophosphazene), then X and Y would both be chlorine. However, X and/or Y may include other substituents such as various organic compounds. PA1 As shown in the above formula, poly(organophosphazenes) are generally prepared by reacting R', which may be one or more organic or organometallic nucleophiles, with poly(dichlorophosphazene). See, for example, Allcock, H. R.; Austin, P. E.; Neenan, T. X.; Sisko, J. T.; Blonsky, P. M.; Shriver, D. F. Macromol., 1986, 19, 1508, and Blonsky, P. M.; Shriver, D. F.; Austin, P. E.; Allcock, H. R. J. Am. Chem. Soc. 1984, 106, 6854. As such, poly(dichlorophosphazene) is an important compound for the synthesis of most other phosphazene polymers. PA1 In the above formula, the representative equation is balanced when n is 3, but based upon multiple molecules of hexachlorocyclotriphosphazene, n may be from about 3 to 10.sup.8. PA1 In the above formula, n may be from about 2 to 10.sup.8. In the condensation polymerization approach, there are some advantages over the ring opening approach. For example, greater molecular weight and polydispersity control are achieved. However, problems including bimodal product molecular weight distributions and relatively low final product molecular weights and use of halogenated hydrocarbons are also realized.
Linear, high polymeric phosphazenes, as represented above, are a class of inorganic polymers which have received extensive study at fundamental and applied levels. Specifically, even though poly(dichlorophosphazene) is a hydrolytically unstable elastomer, it can be converted to a wide range of derivatives by macromolecular nucleophilic substitution reactions with a broad variety of nucleophiles. For example, these polymers are often used in the synthesis of poly(organophosphazenes) through substitution reactions of the parent polymer as shown generally below: ##STR2##
Several methods have been developed to prepare poly(dichlorophosphazenes). Many of these approaches have been developed and mentioned in the literature. However, two methods are the most common as they are the more commercially viable options. The first method is the thermal ring opening polymerization approach. Specifically, a hexachlorocyclotriphosphazene ring (N.sub.3 P.sub.3 Cl.sub.6) is opened as is shown below: ##STR3##
U.S. Pat. No. 4,110,421, discloses a ring opening technique where a sole starting material such as hexachlorocyclotriphosphazene is opened and polymerized in what is described as essentially a melt polymerization technique. This process takes from 5 to 400 hours at from 140 to 250.degree. C. This approach offers some advantages including the simplicity of handling of raw materials. However, there are some facets to this approach which make it less desirable. Those include undesirable waste products expensive and, tedious synthesis and purification of the starting material (hexachlorocyclotriphosphazene), longer periods of time to carry out the reaction, branching, product polydispersity and bimodal molecular weight distributions.
The second commercially viable method is a condensation polymerization approach as shown below: ##STR4##
Based upon these known reactions and accompanying drawbacks, it would be useful to provide a method of preparing phosphazene polymers having a lower product polydispersity, tighter molecular weight controls, shorter reaction times, excellent yield and industrially valuable side products and reduction or elimination of the halogenated hydrocarbon solvents. This may be accomplished by the solid state, single pot, two phase reaction of the present invention which produces poly(dichlorophosphazene). The poly(dichlorophosphazene) may subsequently be reacted with other constituents to form desired polyphosphazenes as is known in the art.